The classical way of transmitting the contents of orthogonally scannable data arrays such as television images is to sample them line by line with pulse-code modulation (PCM) and reorganize the serially arriving PCM samples into a frame at the receiving end. Numerous attempts have been made to reduce the bit rate required for such transmission without sacrificing significant details in the visual representation. Various transcoding techniques designed to reduce the number of bits include linear transformations of the Fourier, Hadamard and other types described, for example, in an article titled Hadamard Transform Image Coding by William K. Pratt, Julius Kane and Harry C. Andrews, Proceedings of the IEEE, Vol. 57, No. 1, January 1969, pages 58-65, and an article titled Image Coding by Linear Transformation and Block Quantization by Ali Habibi and Paul A. Wintz, IEEE Transactions on Communication Technology, Vol. Com-19, No. 1, February 1971, pages 50-63.
By subdividing each array or frame into a multiplicity of elemental data blocks, generally of rectangular shape, it is possible to allocate different numbers of bits to the several blocks in accordance with their relative degrees of activity in terms of signal energy. Reference in this connection may be made to such publications as an article titled Use of "Activity" Classes in Adaptive Transform Image Coding by James I. Gimlett, IEEE Transactions on Communications, July 1975, pages 785 and 786; an article titled Adaptive Coding of Monochrome and Color Images by Wen-Hsium Chen and C. Harrison Smith, IEEE Transactions on Communications, Vol. Com.-25, No. 11, November 1977, pages 1285-1292; and an article titled Adaptive Transform Image Coding for Human Analysis by O. R. Mitchell and Ali Tabatabai, ICC 1979, Boston, pages 23.2.1-23.2.5.
Though the idea of adaptive coding is sound, the techniques hitherto proposed have various drawbacks. Thus, there is no known way of determining "activity" in a manner which would definitely reduce the reconstruction error of each block--i.e. the distortion inherent in the limitation of the bit allocation--to a tolerable value avoiding the loss of significant details. The utilization of a necessarily small number of empirically established bit-assignment tables (four of them in the systems described in the articles referred to) cannot provide an optimum bit allocation for each block. Moreover, selection of the table best suited to minimize the reconstruction error would require a successive testing of all available tables in each instance.
A paper titled Encoding of a Counting Rate Source with Orthogonal Functions by V. R. Algazi and D. J. Sakrison, presented at the Symposium on Computer Processing in Communications held Apr. 8-10, 1969 at the Polytechnic Institute of Brooklyn, New York (published 1970 in Symposia Series, Vol. 19 of Microwave Research Institute, pages 85-99), discusses varying the law of quantizing for the purpose of coding a given coefficient with the least number of quantum steps (i.e. the largest separation of quantization levels) compatible with a permissible rate of distortion; according to this paper, a Huffman coder can be used for ascribing shorter binary sequences to the more roughly quantized levels. These authors, however, do not suggest the use of such different quantizing laws for the adaptive coding of data blocks.